Calculate the work done by the brakes of a car of mass 1000 kg when its speed is reduced from 20 m /s to 10 m /s?
Mass of the car, m = 1000kg
Initial velocity (v1) = 20m/s
Final velocity (v2) = 10m/s
Initial kinetic energy = 1/2 mv12
= 1/2 x 1000 x (20)2
= 200,000J
Final kinetic energy=1/2 mv22
= 1/2 x 1000 x (10)2
= 50,000 J
Work done by the brakes = change in kinetic energy
= (K.E) 2 - (K.E) 1
= 50,000 - 200,000
= - 150000J
= - 150KJ
(The negative sign shows that the force applied by the breaks is in the opposite direction to the direction of motion of the body)